SPATIAL VARIATION ABLATION ON GLACIERS AND ALPINE AREAS, NORTH CASCADES, WASHINGTON  

Mauri S. Pelto, Dept. of Environmental Science,

Nichols College, Dudley MA 01571 peltoms@nichols.edu

 

 ABSTRACT

                In the North Cascade Range, Washington snowpack accumulation and the resultant ablation provides critical summer water resources.  Utilizing SWE data from 10 USDA Snotel sites and 13 glaciers in the North Cascades the variation in maximum SWE accumulation and ablation rates with location are analyzed.  Maximum SWE at the glacier locations is 299% and 212% of that at the low elevation and high elevation Snotel sites.  

Ablation has been measured for periods of at least two weeks with on site temperature measurement at numerous Snotel and glacier sites.  The resulting relationship is consistent and indicates that on-site temperature provides a good estimate of ablation over a multi-week period regardless of location at a Snotel site or on a glacier.  This holds true for snow and for glacier ice, though the relationships are different.  Ablation peaks in May at low elevation Snotel sites, in June at high elevation Snotel sites, and in August on glaciers.  Glacier dominate runoff from the alpine zones from July-August.  This indicates that monitoring of glacier ablation is essential for forecasting of alpine runoff.  The regional nature of both the ablation-temperature and accumulation-Diablo Dam precipitation relationships indicates that neither is particularly dependent on microclimates.   It further suggests that once ground truth data is available for a limited duration for a site, maximum SWE or ablation could be determined from climate data.

 

INTRODUCTION

                The spatial and temporal variation of snowpack accumulation, snowpack ablation and consequent alpine runoff is crucial to determining regional summer water resources in the North Cascades Range, Washington.  Glaciers alone provide 750 million m3 of runoff each summer (Fountain and Tangborn, 1985).  What is the spatial and temporal variability of accumulation and ablation across the North Cascades?  Rasmussen and Tangborn (1976) noted a poor relationship between observed annual precipitation and annual runoff.  They also noted, in plotting mean annual runoff versus basin mean altitude in 36 basins in the North Cascades, that there was a poor relation between runoff and basin altitude.    

These observations demonstrate that precipitation variation is complex and significant in the region and that extrapolations cannot be made from a standard measurement site to a secondary location based simply on elevation change.  That further extrapolations could not be made from site to site with the available datq.  This paper examines whether maximum winter season snowpack, and snowpack and glacier ablation, can be determined for secondary locations from standard locations once baseline data exists for the secondary sites. 

 

DATA SETS

The following data sets are used (Table 1 and Figure 1): 1) Annual glacier mass balance measurements from thirteen North Cascade glaciers (NCGCP on 8; USGS on 1 and NPS on 4). 2) Snowpack ablation from 1984-2002 on four glaciers (3 by NCGCP and 1 by USGS). 3) Daily snow water equivalent and temperature data from 10 USDA Snotel sites.

The Natural Resources Conservation Service (NRCS) of the US Department of Agriculture (USDA) operates an extensive automated Snotel system to collect snowpack and climatic data in the western United States.  Snotel sites have at a minimum a pressure sensing snow pillow, storage precipitation gage and air temperature sensor.  The snow pillows are envelopes of stainless steel or synthetic rubber containing an antifreeze solution.  As snow accumulates on the pillow it exerts a pressure that is measured and converted to a reading of snow water equivalent and telemetered to two NRCS master stations.  Each site measures snow water equivalent (SWE) maximum, minimum and average daily temperature (Figure 1).   

From 1000-1900 m the USDA Snotel network provides an excellent network of snowpack and temperature data recorders in the North Cascades, but no sites are found on or adjacent to the highest accumulation areas, which are glaciers.   The necessity of using Snotel sites and glaciers to adequately identify snowpack water resources in the North Cascades is emphasized by the difference in mean maximum winter accumulation in SWE from 1.17 m at the ten USDA Snotel sites, ranging in altitude from 1000-1900 m, and 2.93m at nine glacier locations ranging from 1650-2200 m.  By July 15 the nine glacier locations still average 1.3 m SWE, while the Snotel sites have no snowpack remaining.  Thus, Snotel sites provide a good indicator of late spring and early summer runoff and glaciers a better measure of mid and late summer runoff. 

Mass balance measurements have been made using the same methods at the same time of the year on nine North Cascade glaciers by NCGCP (Pelto, 1996; and Pelto and Riedel, 2001).  The USGS has maintained a mass balance record and weather records at South Cascade Glacier since 1958 (Krimmel,1993-1999).  The North Cascades National Park Service began measuring mass balance on four glacier in 1993 (Pelto and Riedel, 2001). Each program monitors ablation during specific time periods using stakes emplaced in the glacier surface.  Revisiting each site through the ablation season and measuring the emergence of each stake identifies the ablation rate.  The maximum snowpack depth and water equivalent is also determined at specific locations at approximately the same time each year in early to mid-May utilizing probes driven through the snowpack on the glacier. 

 

SNOWPACK ABLATION

 

Early in the melt season (April-June 15), ablation is dominated by melt at the lower elevation range (>1500m) in alpine basins (Pelto, 1996; Fountain and Tangborn, 1985).   Ablation during May at Snotel sites from 1000-1500 m, averages 0.018 m/day, while at sites from 1500-1900 m average ablation is 0.012m/day, and above 1600 m on glaciers average ablation is 0.08 m/day (Table 3).  Snowpack ablation is reduced somewhat for the Snotel sites are that more protected by surrounding forest (Wells Creek and Thunder Basin).  

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Observations of snowpack loss at Snotel sites in the North Cascades identifies the variations in ablation.  The characteristics of sites used are noted in Table 1, Fish Lake, Harts Pass, Lyman Lake, Miners Ridge, Park Creek, Rainy Pass, Stampede Pass, Stevens Pass, Thunder Basin and Wells Creek. Snowpack is lost from the lower Snotel sites, below 1400 m,  in May or early June.   The early ablation season, is marked by freezing levels that frequently result in snowfall at Lyman Lake and rainfall at the lower elevation Snotel sites.   Cross correlation of May monthly ablation rates between Snotel sites as a result is poor, as are the daily ablation rates, correlation coefficients ranging from 0.43-0.76.   Correlating ablation at Lyman Lake with precipitation, maximum, minimum and average temperature, indicates the best correlation coefficient for monthly May ablation is with average daily temperature at 0.74.  The early season monthly correlation between the high elevation Snotel sites and glacier sites is modest at 0.72.

Average ablation after June 1 is limited to data from Snotel sites above 1400 m and glaciers.  At the three stations, where snowpack endured throughout all of June, ablation ranged from 0.027 -0.032 m m/day.  June ablation on South Cascade, Easton and Columbia Glacier, during these same June periods, ranged from 0.23-0.29 m/day, averaging 0.027 m/day.  The correlation from glacier to glacier for the same time periods is 0.86-0.99, indicating that ablation conditions are becoming increasingly consistent on glaciers as the summer melt season develops.  Correlation in daily ablation rates for the three Snotel sites is 0.79-0.92   indicating that in the elevation zone from 1500-2000 m across the North Cascades ablation after June 1 has a comparatively low degree of variability.  Correlating ablation at Lyman Lake with precipitation, maximum, minimum and average temperature, indicates the best correlation coefficient for June is with maximum daily temperature at 0.85.   

By early July snowpack beyond the glacier margins is limited, Snotel sites have lost their snowcover, and yet streamflow is still heavily dependent on snow and ice melt from glaciers (Fountain and Tangborn, 1985; Pelto, 1996). From July-September glaciers are the primary area of residual snow and ice ablation.  This region has the highest melt rates during this period, while other inputs are at an annual low (Rasmussen and Tangborn, 1976).  Thus, glaciers ameliorate low flow conditions (Fountain and Tangborn, 1985; Pelto, 1993).   In heavily glaciated basins such a Baker River from 20-45% of the total input is from glacier melt during the latter part of the summer (Pelto, 1996; Post et al; 1971).  This glacier runoff is best determined by direct measurement of ablation on glaciers.  NCGCP  (Pelto, 1996; Pelto and Riedel, 2001) and the USGS (Krimmel, 1998) measurements on glaciers do provide a direct measure of ablation in this elevation band at multiple locations over the last 20 years.   Stakes drilled into the snow and ice of the glacierís are measured several weeks and or months after emplacement.  This provides the ablation rate.

Ablation measurement on nine North Cascade glaciers for twenty-nine discrete two to six week periods during this part of the ablation season yield mean ablation rates of 0.036 m/day, 0.038 m/day and 0.028 m/day for July, August and September respectively.  The correlation in mid and late season ablation between each glacier exceeds 0.95 indicating the degree to which the regional summer climate is consistent across the North Cascades. 

Comparison of ablation rates and onsite temperature records in the case of the South Cascade Glacier, Easton Glacier, Ice Worm Glacier and Columbia Glacier yield a relationship between air temperature and daily ablation for snow and ice in SWE (Figure 1).   Figure 3 also contains data from Lyman Lake, and Stevens Pass.  The ablation data-temperature relationships is not statistically different for the Snotel and glacier sites.  There is a significant difference between snow and ice ablation for the same temperature (Figure 1).

 

 

  CONCLUSIONS

Accumulation is widely variable and can only be estimated if baseline data is available.  Data from Lyman Lake and Diablo Dam provide the best overall correlation for maximum SWE.  Ablation rates in May at the start of the melt season are widely variable from site to site, but fit within specific mean ranges based on elevation.  Ablation rates after June 1 are similar in the summer season, and can be extrapolated from primary to secondary sites in regions above 1500 m, without substantial baseline data.  By mid-summer ablation rates do not vary substantially within the 1600 m-2400 m elevation band, which is the primary elevation zone for glaciers.  The most important ramification is that if the distribution and depth of the snowpack is known on June 1, than summer water resources can be estimated for a wide range of basins from a limited number of primary ablation measurement sites.

Local climate in the North Cascades influences mean snowpack depth and ablation rate, but does not cause significantly different responses to annual climate conditions within specific elevation bands.   Extrapolation from site to site for accumulation can be accomplished, but only when the sites are at similar elevations and the sites have a baseline history documenting the specific development of snowpack.  .

To model or directly calculate the timing and magnitude of water resource storage it is essential to collect baseline data on accumulation at numerous secondary sites.  Once the relationship of these secondary sites can be related to long-term records at primary measurement sites, then the secondary sites measurements can be discontinued.  This also applies to early season, April-May ablation.   Ablation rates and consequent runoff can be assessed from a few primary sites at glaciated levels from June 1-September 31.   It is also evident that the Snotel system provides an excellent and cost effective means of collecting data on snowpack development from 1000-1900 m in the North Cascades, but does not well represent snowpack accumulation at the average glacier accumulation zones of 2000 m.  Making accurate summer streamflow estimates is impossible without data from glacier sites.

REFERENCES

Fountain, A and Tangborn, W.V. 1985. 'The effect of glaciers on  streamflow variations'.  Water Res. Res., 21, 579-586.

Krimmel, R.M.  1993.  'Mass balance, meteorologic, and runoff measurements at South Cascade Glacier, Washington, 1992 balance year'. USGS OFR-93-640.

Krimmel, R.M.  1994.  'Runoff, Precipitation, mass balance, and ice velocity measurements at South Cascade Glacier, Washington, 1993 balance year'.   USGS  OFR-94-4139.

Krimmel, R.M.  1995.  'Water, ice and meteorological measurements at South Cascade Glacier, Washington, 1994 balance year'.  USGS OFR-95-4162.

Krimmel, R.M.  1996.  'Water, ice and meteorological measurements at South  Cascade Glacier, Washington, 1995 balance year'.  USGS OFR-96-4174.

Krimmel, R.M.  1997.  'Water, ice and meteorological measurements at South Cascade Glacier, Washington, 1996 balance year'.  USGS OFR-97-4143.

Letreguilly, A. and Reynaud. L. 1989.  Spatial patterns of mass balance fluctuations of North American glaciers.  J Glaciol.,  35(120), 163-168.

Pelto, M.S.  1993. 'Current behavior of glaciers in the North Cascade and effect on regional water supplies'. Washington Geology, 21(2), 3-10.

Pelto, M.S.  1996.  'Annual balance of North Cascade glaciers from 1984-1994'. J. of Glaciology, 41, 3-9.

Pelto, M.S. 1996.  'Recent changes in glacier and alpine runoff in the North Cascades, Washington'.  Hydrol. Processes, 10, 1173-1180.

Pelto, M.S. and Hedlund, C. 2001.  Terminus behavior and response time of North Cascade glaciers, Washington U.S.A. Journal of Glaciology 47, 497-506.

Pelto, M.S. and Riedel, J. 2001.  Spatial and temporal variations in annual balance of North Cascade glaciers, Washington 1984-2000.  Hydrologic Processes.

Rasmussen. L.A., and Tangborn, W.V. 1976.  'Hydrology of the North Cascade Region, Washington 1.  Runoff, Precipitation, and Storage Characteristics'.     Wat. Res. Res., 12(2), 187-202.


 

Elevation

Lati-tude

Long-itude

Source

Lyman Lake

1805

48 12

120 55

USDA

Rainy Pass

1460

48 33

120 43

USDA

Thunder Basin

1285

48 31

120 59

USDA

Stevens Pass

1245

47 44

121 05

USDA

Stampede Pass

1190

47 17

121 20

USDA

Wells Creek

1280

48 51

121 47

USDA

Park Creek Ridge

1405

48 27

120 55

USDA

Fish Lake

1030

47 31

121 04

USDA

Miners Ridge

1890

48 10

120 59

USDA

Columbia Glacier

1450-1750

47 58

121 21

NCGCP

Daniels Glacier

2000-2250

47 34

121 10

NCGCP

Ice Worm Glacier

1900-2050

47 34

121 10

NCGCP

Lynch Glacier

1950-2250

47 34

121 11

NCGCP

Rainbow Glacier

1350-2250

48 48

121 40

NCGCP

Easton Glacier

1700-2900

48 44

121 50

NCGCP

S.Cascade  Glacier

1645-2100

48 21

121 03

USGS

L.Curtis Glacier

1650-1950

48 50

121 37

NCGCP

Neve Glacier

1830-2150

48 39

121 08

NCGCP

Cache Col Glacier

1880-2100

48 22

121 03

NCGCP

 

Table 1.  Location of USDA snotel, USGS and NCGCP glacier

measurements sites.

 

Site

Maximum SWE

Maximum Date

May Ablation

Fish Lake

0.83

4/5

0.019

Harts Pass

1.17

5/1

0.014

Lyman Lake

1.63

5/10

0.012

Miners Ridge

1.3754

5/10

0.012

Park Creek

1.12

4/10

0.023

Rainy Pass

1.04

4/15

0.018

Stampede Pass

1.19

4/10

0.021

Stevens Pass

1.07

4/5

0.019

Thunder Basin

0.84

4/15

0.014

Wells Creek

0.79

4/15

0.015

 Table 2.  The average maximum SWE,, average date of maximum SWE, and mean daily ablation in May at Snotel sites. 

 

May June Maximum Date Ablation Season Begins elevation
Fish Lake 23.4 33 0.8382 5-Apr 1030
Harts Pass 16.8 34.3 46 1.1684 1-May 1905
Lyman Lake 14.8 38.3 64 1.6256 10-May 1805
Miners Ridge 14.7 31.1 54 1.3716 10-May 1890
Park Creek 28.6 44 1.1176 10-Apr 1405
Rainy Pass 22 41 1.0414 15-Apr 1460
Stampede Pass 26.2 47 1.1938 10-Apr 1190
Stevens Pass 22.8 42 1.0668 5-Apr 1245
Thunder Basin 16.8 33 0.8382 15-Apr 1285
Wells Creek 17.8 31 0.7874 15-Apr 1280
South Cascade 114 2.8956 1915
Easton 160 4.064 2200
Lynch 150 3.81 2200
Columbia 120 3.048 1650

Table 3 Ablation rates per  month at different snotel sites.  

   

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